This invention relates to optical waveguides and more particularly to a method of making a high numerical aperture gradient index waveguide.
Waveguides used in optical communication systems are herein referred to as "optical waveguides," and are normally constructed from a transparent dielectric material, such as glass or plastic.
Gradient index optical waveguides have a radially varying composition and consequently a radially varying refractive index. These waveguides focus light propagated axially along them. This focusing effect is described as a high numerical aperture. U.S. Pat. No. 3,823,995, Carpenter and U.S. Pat. No. 3,826,560, Schultz, describe techniques for making gradient index optical waveguides.
The aforementioned Schultz patent describes a flame hydrolysis technique for making a glass preform wherein the concentration of a dopant added to a glass forming silicon compound is changed as the preform is built-up radially. Changing concentration of the dopant changes the refractive index in the desired manner.
Optical waveguides having high numerical aperture values (approximately 0.20 or more) have a sharp radial composition gradient which causes stress in the waveguide. These stresses arise from the variation in the thermal expansion coefficient introduced by the radial variation in composition. Also, in outside vapor phase oxidation processes, such as that described in the aforementioned Schultz patent, removal of the starting member from the preform results in a small hole at the center. Residual tension in the waveguide has its highest value at the surface of this hole. All of these stresses cause fracture and failure when the preform is sintered into an optical waveguide preform and cooled. It is desirable to reduce these stresses.